Process for the manufacture of desulfurizing agents for crude iron or steel melts

ABSTRACT

The present invention relates to a process for the manufacture of desulfurizing agents containing 1-6% by weight of chemically bonded water, based on calcium oxide-containing calcium carbide, for crude iron and steel melts. To this end, a molten mixture of calcium carbide and calcium oxide with a CaO content of 20 to 80% by weight is produced, which is allowed to solidify into a block. Next, the block which still has an average temperature of more than 400° C., is rough-crushed to particles with a size of less than 150 mm and calcium oxide is added to the comminuted mixture, which still has a temperature of at least 400° C., in accordance with the CaO content desired in the end product. The mixture is finally ground in the presence of air or nitrogen with a moisture content of from 5 to 20 g/m 3  (at 1.013 bar and 273.15 K) at temperatures below 100° C. to particles with a size of less than 10 mm.

The present invention relates to a process for the manufacture ofdesulfurizing agents containing 1-6% by weight of chemically bondedwater, based on calcium oxide-containing calcium carbide, for crude ironand steel melts.

The desulfurization of crude iron and steel melts by means of calciumcarbide (referred to hereinafter as carbide) containing calcium oxide(briefly referred to hereinafter as lime) is generally known (DE-PS No.1 160 457, DE-PS No. 20 37 758).

Heretofore, it has been customary to prepare these desulfurizing agentsby processes, wherein the required amount of finely divided lime wasintroduced into molten carbide in order to produce a homogeneous mixturein the melt, which was then allowed to cool and was comminuted.

Preferably, the finely divided lime was introduced into the stream ofcarbide tapped off from the furnace. Although the amount of lime thatcan be introduced into the carbide melt is limited and there are risksassociated with working with liquid carbide, it has long been held inthe art that this method could not be avoided since it was thought thatonly a mixture of CaC₂ and CaO produced in the melt was ideally suitedfor desulfurization of metal melts.

Desulfurizing agents based on calcium carbide that contain substanceswhich split off water at the temperature of the metal melt have alreadybeen described in DE-AS No. 22 52 795. These agents, which are mixturesof customary commercial carbide with, for example, Ca(OH)₂ as thesubstance that splits off water, have the disadvantage that they aremixtures produced by mechanical mixing processes in which carbideparticles exist separately from the Ca(OH)₂ particles, so that the useof these products leads to higher consumption, irregular and vigorousgas reactions and a large variation as regards the desulfurizingefficiency, rendering difficult any controlled use of these agents.

The present invention now unexpectedly provides a process for making ahighly effective desulfurizing agent for crude iron and steel meltswhich process permits the disadvantages of prior art methods to beavoided and which comprises: thermally producing from lime and coke amolten starting mixture of carbide and calcium oxide with a CaO contentvarying within the range 20 to 80%; allowing the mixture to cool andsolidify into a block; rough-crushing the solidified block while itstill has an average temperature of more than 400° C., preferably ofbetween 400° C. and the solidification temperature of the melt, toparticles with a size of less than 150 mm and calcium oxide; andadmixing the comminuted mixture, which still has a temperature of atleast 400° C., with a quantity of calcium oxide, necessary to establishin the mixture a total content of CaO corresponding to the CaO contentdesired in the end product. Preferably, calcium oxide is added in aquantity necessary to establish in the mixture a total content of CaO ofmore than 45% up to 90% by weight. Next, the mixture is ground withintensive mixing and in the presence of air or nitrogen having amoisture content of 5 to 20 g/m³ (at 1.013 bar and 273.15 K) attemperatures below 100° C., preferably at 10°-50° C., to particles witha size of less than 10 mm, preferably of less than 100 μm.

A preferred feature of the present process provides for the moltenstarting mixture of calcium carbide and calcium oxide to contain 20 to45% by weight of CaO, the mixture having been obtained from lime andcoke in known manner by thermal treatment. It is also possible, however,initially to produce a molten starting mixture of calcium carbide andcalcium oxide with a CaO content of more than 45% up to 80% by weight,by introducing finely divided calcium oxide into an existing calciumcarbide melt containing up to 45% by weight of CaO until establishmentof a maximum CaO content of up to 80% by weight, then allowing the wholeto solidify into a block, and rough-crushing the latter at temperaturesof above 400° C.

The present process permits avoiding the mixing of the lime with acarbide melt and the difficulties associated therewith. In addition tothis, the present process has, inter alia, the following beneficialeffects: It is not necessary for the composition of the burden to be setin each particular case for producing the carbide melt, nor is itnecessary for the lime to be first ground to a certain particle size.Use can rather be made of a carbide block with a CaC₂ :CaO weight ratiowhich may vary within wide limits. In other words, the carbide block maycontain CaC₂ and CaO in practically any ratio, and the lime can even beused in the form of coarse particles with a size within the range 8 to60 mm, for example.

A further beneficial effect of the agent produced in accordance withthis invention resides in the fact that on the surface each individualparticle of Ca(OH)₂ lies close to CaC₂, with the result that thedesulfurizing reaction is started very early and regularly. As a resultof this, smaller amounts of desulfurizing agents are required to be usedfor producing comparable desulfurization results, which incidentally arecontrollable.

The following Examples illustrate the invention:

EXAMPLE 1

The starting material was a melt as customarily used for the manufactureof commercial carbide, the melt containing 80% by weight of CaC₂ and 20%by weight of CaO. A carbide block of corresponding composition wasproduced in known manner in a crucible by allowing this melt to cool.

After the block had cooled to an average temperature of approximately600° C., it was rough-crushed to particles with a size of less than 150mm and the carbide, still with a temperature of 500° C., was coveredwith a layer of sufficient lime with a particle size of 8 to 60 mm thatthe resulting mixture had a total CaO content of 50% by weight.

Next, the mixture was ground in a rotary mill, while 1500 m³ /h of airwith a moisture content of 10 g/m³ (at 15° C.) was passed therethrough.The throughput was 500 kg/h at 50° C., and the mixture was ground toparticles with a size of up to 100 μm.

1500 kg of this product was used for customary desulfurization of 300000 kg of a crude iron melt that contained 0.03% by weight of sulfur.The iron so treated contained 0.005% by weight.

EXAMPLE 2

Calcium carbide was thermally produced in known manner from lime andcoke, the lime/coke mixture in the total burden being set at a weightratio of approximately 110:40, corresponding to a carbide with a CaOcontent of approximately 45% by weight.

CaO of a particle size of 3-8 mm was metered into the carbide tappedoff, the CaO being used in a quantity necessary to establish an averagecontent of approximately 80% by weight of CaO in the discharge crucible(approximately 1.2-1.3 t of CaO per metric ton material tapped off).

After cooling the crucible to an average temperature of no less than600° C., which was the case after 4 hours, the block was rough-crushedto particles with a size of less than 150 mm and a layer of sufficientlime of particles with a size of 8 to 60 mm was added to the hot mixturethat the average CaO content was 90% by weight. Next, the mixture wasground in a rotary mill, while 1500 m³ /h of air with a moisture contentof 10 g/m³ (at 15° C.) was passed therethrough. The throughput was 500kg/h at 50° C. and the material was ground to particles with a size ofless than 100 μm.

The mixture permitted the same desulfurization results to be produced,based on the calcium carbide content, as described in Example 1.

We claim:
 1. In the process for the manufacture of desulfurizing agentsconsisting essentially of calcium carbide and calcium oxide for crudeiron and steel melts, the improvement which comprises: producing fromlime and coke a molten mixture of calcium carbide and calcium oxidehaving a CaO content varying within the range 20 to 80% by weight, therest being essentially calcium carbide; allowing the mixture to cool andsolidify into a block; rough-crushing the solidified block while itstill has an average temperature of more than 400° C. to particles witha size of less than 150 mm; admixing the comminuted mixture, which stillhas a temperature of at least 400° C., with a quantity of calcium oxidenecessary to establish in the resulting mixture a total content of CaOcorresponding to the CaO content of up to 90% by weight desired in theend product, and then grinding the mixture with thorough agitation attemperatures below 100° C. to particles with a size of less than 10 mm;contacting the mixture while grinding with a sufficient amount of air ornitrogen having a water content of 5 to 20 g/m³ (at 1.013 bar and 273.15K) until 1-6% by weight of water, related to the end product, ischemically bonded by hydrating a quantivalence of CaO to Ca(OH)₂.
 2. Theprocess as claimed in claim 1, wherein coarse particles of CaO with asize of 8 to 60 mm are added to the rough-crushed mixture.
 3. Theprocess as claimed in claim 1, wherein the mixture is crushed attemperatures of between 400° C. and the solidification temperature. 4.The process as claimed in claim 1, wherein the molten starting mixtureof calcium carbide and calcium oxide with a CaO content of 20 to 45% byweight is produced from lime and coke in customary manner by thermaltreatment.
 5. The process as claimed in claim 1, wherein the moltenstarting mixture of calcium carbide and calcium oxide with a CaO contentof more than 45% up to 80% by weight is produced by introducing finelydivided calcium oxide into an existing calcium carbide melt containingup to 45% by weight of CaO until establishment therein of a maximum CaOcontent of up to 80% by weight and then allowing the whole to solidifyinto a block.
 6. The process as claimed in claim 1, wherein the mixtureis ground at 10°-50° C. to particles with a size of less than 10 mm. 7.The process as claimed in claim 1, wherein the mixture is ground toparticles with a size of less than 100 μm.